Before, it was a phrase to notify someone not to get involved in whatever the situation is or what you're talking about. Now, it basically is apart of poor defense tactics used when a person abruptly joins your conversation, has their say and humiliates you by showing how wrong you are. Commonly used by actors in the show What Would You Do.

Boyfriend: Now get back inside, try on some clothes that makes you look more hot. NOW!!

Old Woman: Hey! Your girlfriend can wear what she likes! Your just being rude!!!